1. Field of the Invention
The present invention relates to an optical lens made of plastic, which is used in various kinds of optical instruments.
2. Related Background Art
Optical lenses made of plastic, such as the one shown in FIGS. 4 and 5, have conventionally been known and widely used for eyepieces of 8-mm video cameras, objective lenses of CD-ROM drives, and the like. The optical lens 101 of FIGS. 4 and 5 is made by injection molding, transfer molding, or the like using dies. For making the optical lens 101, a molten resin such as PMMA (polymethyl methacrylate) is caused to flow into a cavity from a gate. Then, the resin is solidified by cooling. After the resin is solidified, the part of resin solidified within the gate orifice, or both the gate and the part of resin solidified within the gate orifice (hereinafter referred to as "residual gate part" collectively) are removed, and surface treatment is carried out as required. Thus, the optical lens 101 is completed.
For processing the above-mentioned residual gate part, it will essentially be ideal if the residual gate part 110 is removed alone from the optical lens 101 by moving a cutting tool such as end mill along the outer circumferential face 104 of the optical lens 101. In general, however, in view of the processing accuracy of the processing machine, reduction in manufacturing cost, and the like, a part of the outer circumferential part (flange part 106) of the optical lens 101 is cut flat as shown in FIG. 4, thereby removing the residual gate part 110. Consequently, the outer circumferential face 104 of the optical lens 101 completed as a product includes a flat, gate-removed part 105 which is defined when the residual gate part 110 is removed.
On the other hand, in general, an optical lens made of plastic absorbs therein moisture (water) existing in the air, due to the hygroscopicity inherent in the plastic material. Also, it is known that the moisture is absorbed into the optical lens 101 radially from the outer circumferential face 104 of the optical lens 101 toward the optical axis (a) thereof.